One of the more important aspects in managing the lower half in the swing sequence is understanding how to work with and against the rear leg. This is a mechanic that is used on a frequent basis as we see loading patterns of elite hitters and strikers all embodying these movements in some capacity. A physical "cue" that we use at our academy is, "load against, not over" the rear leg. What this allows the hitter to do is put themselves in a position to activate and prepare the rear hip to begin the swing sequence.
This cue tends to over-correct the hitters desire to lean and not load when "staying back". Often times when hitters, especially amateur hitters process the loading movement they begin to sway their body back onto the top of their rear leg and into the quadriceps/knee. This knee dominant load takes the load from the rear hip and into a resting position. In an efficient sequence, the load must work up through the ground, hips, torso, upper extremities and out through the barrel of the bat. When we load and shift our weight over the rear leg and knee, that leads to a process that is much more inefficient from a weight transfer and timing perspective. Often, these hitters compound the error by either remaining stuck on their rear leg and spinning or "jumping" forward in an effort to get back to a more neutral, athletic position.
One of the things we can take notice to in an efficient loading sequence is the action of the rear leg. Looking at the two baseball swings in the above images, you can see that as the hitters load against the rear hip/leg, the back knee never makes it's way to the outside part of the foot. When the hitter (or striker as seen in the golf swing to the right) begins to initiate the swing sequence, the rear hip turns the rear leg (femur) internally which causes the back knee to pivot and send the sequence down through the release of the toe. When we talk about the release of the rear leg, if there is too much emphasis placed on the rear knee/foot we end up with a spinning effect that lacks power and direction.
Again, take note to the where the energy is being transferred through the lower body. We see the "pinching effect" created as the strikers feet pull through the ground during the rotation. This leads to an efficient "snap" of the rear hip and a bracing effect of the front leg. You can see the hitters and strikers lead foot pulling rearward with the lead leg lengthening to accelerate the rear hip/leg. With the baseball players in the above clips, we see similar actions of the hips and effects of the lead leg (this being a more advanced and detailed topic for later posts).